1. Field of the Invention
The invention relates to a power supply circuit.
2. Related Art
In semiconductor devices such as a system LSI, it has become difficult to commonly use a fixed power voltage for its entire system. For such a semiconductor device, a power supply circuit is required that converts a power voltage to output the power. Some of the power supply circuits have a standby function to reduce power consumption. The standby function is a function that cuts a DC current by inactivating the power supply circuit.
The power supply circuit having the standby function includes two voltage converting circuits that are connected in parallel to each other and a reference-voltage generating circuit (also referred to as BGR (Band Gap Reference) circuit). These two voltage converting circuits are connected between the BGR circuit and an output terminal.
The BGR circuit receives an external voltage and outputs a reference voltage. The two voltage converting circuits convert the external voltage into an internal power voltage by reducing or boosting the external voltage based on the reference voltage, and outputs this internal power voltage. The internal power voltage is output from the output terminal of the power supply circuit, and is used as a power voltage in a semiconductor device.
One of the two voltage converting circuits (first voltage converting circuit) is active at all times, and the other one of the two voltage converting circuits (second voltage converting circuit) is controlled to be active or inactive by a control signal. When the second voltage converting circuit is active, the internal power voltage is a voltage that is determined by the first and the second voltage converting circuits. On the other hand, when the second voltage converting circuit is inactive, the internal power voltage is a voltage that is determined by the first voltage converting circuit active at all times.
In a conventional power supply circuit, when the second voltage converting circuit transits from an inactive state to an active state, the internal power voltage is deviated from a desirable voltage, and it takes time to recover a steady state. That is, a transition response time is long when the second voltage converting circuit transits from the inactive state to the active state. This is because RC delay is caused due to a parasitic capacitance and a resistor of wirings arranged in the power supply circuit.